Wetlands are transitional areas between water and land and have the characteristics of both terrestrial and aquatic systems. The term wetlands means those areas that are saturated by surface or groundwater at a frequency and duration to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions.

Wetlands cover approximately 6% of the earth’s surface. In Pakistan there are 200 wetlands and among them 19 are globally important and placed in the Ramsar list. There are two types of wetlands i.e., natural wetlands and constructed wet lands. Natural wetlands are variously called as swamps, marshes, bogs, fens, wet meadows, and sloughs. Natural wetlands are natural receptacles. As natural wetlands are present in low lying areas, these receive runoff water and overflow from rivers and streams. In response, various wetlands act as biological mechanisms or processes evolved over geologic time to treat inflows. These mechanisms trap sediments and breakdown a wide range of pollutants into elemental compounds.

Wetlands have a natural ability to treat wastewater. Water moves slowly through wetlands, as shallow flows, saturated substrates or both. Slow flows and shallow water cause sediments to settle. The slow flows also act to prolonge contact times between the water and surfaces within the wetland. The organic and inorganic materials within a wetland form a complex mass. This mass along with the occurance of gas/water interchanges promotes a variety of substances.

Dense growth of vascular plants adapted to saturated conditions often thrive in wetlands and contribute to its treatment capacity. Along with slowing the flow of water, the vegetation creats microenvironments and provides the microbial community attachment sites. Further plants die back in the fall and accumulate as litter. This creates additional material and exchange sites as well as providing a source of carbon, nitrogen and phosphorus to fuel microbial processes.